Drill bits utilizing rotary cones for earth boring operations are well known in the art of drilling. The bits generally include a threaded upper portion that attaches to a drill string and a body portion with three downwardly and inwardly facing support spindles. Each support spindle consists of a cylindrical base pin and a smaller, cylindrical pilot pin further projecting along the longitudinal axis of the spindle. A cutting cone is rotatably mounted on each of the support spindles. Each cutting cone includes spaced rows of cutting teeth distributed around the outer surface of the cone.
During operation of an earth boring drill bit, the weight of the drill string places a load on the lower face of the cutting cone. The load generally causes contact between an inner surface of the cutting cone and a surface of the support spindle. The friction resulting from this contact between the rotating cutting cone and the stationary support spindle causes wear on the contacting surfaces that limits the useful life of the drill bit. To combat this problem, many bits use lubricant on the contacting surfaces between the support spindle and the cutting cone to slow the rate of surface wear. Drill bits or prior designs, however, prevent uniform lubrication of the spindle and causes some parts of the spindle to wear out more rapidly than others.
In a drill bit of a prior design, the load generally causes contact between an inner surface of the cutting cone and a surface of the support spindle on the lower, or load, side. The load also causes a corresponding gap between the inner surface of the cutting cone and a surface of the support spindle on the upper, or non-load, side. To maintain lubrication of the spindle, conventional bits rely on a process by which the rotation of the cone carries lubricant from the gap on the non-load side of the spindle to the contacting surfaces on the load side of the spindle.
The exact location of the contact between the spindle and cutting cone surfaces depends on the location of the load applied to the bit. Earth boring bits operate in two basic modes. Most cutting cones are designed so that the load is primarily applied to the outer one or two rows of cutting teeth. Bits with cutting cones of this type operate in a "cocked" mode. Cutting cones that are designed so that the load is applied closer to the centerline of the bit body operate in a "normal" mode. The operative mode is determined by the location of the load applied to the cutting cone when engaging rock at the hole bottom. The location of the load applied to the cone is a function of the cone design. In both the "normal" and "cocked" mode, the rotation of the cutting cone delivers sufficient lubricant to the contacting surfaces of the base pin and pilot pin to provide effective lubrication of those surfaces.
However, conventional drill bits also include a flat thrust face at the transition between the base pin and the pilot pin for supporting axial loads applied to the cutting cone. When the cutting cones of these conventional bits are loaded in the "normal" mode, the axial component of the load causes a radial inner surface of the cutting cone to substantially contact the entire thrust face surface. When the bits operate in a "cocked" mode, there is a gap on the non-load side of the thrust face, although this gap is smaller than the gaps on the lateral surfaces of the spindle. The reduced size or absence of a gap on the non-load side reduces the ability of the rotating cutting cone to carry lubricant to the load side. In either the "normal" or "cocked" mode, therefore, the thrust face does not lubricate as efficiently as do the lateral surfaces of the spindle. The lack of lubrication on the thrust face increases heat generated by friction thereby promoting galling of the spindle and often causing premature failure of the spindle. Consequently, the useful life of the drill bit is limited by the inability to maintain sufficient lubrication of the spindle thrust face. The present invention addresses these friction-related problems by shaping the support spindle to promote lubrication of the thrust face.